1. Field of the Invention
This invention relates to the art of thermal transfer printing and more particularly, to a dye for cyan color which is contained in an ink layer of a thermal transfer sheet for use as a recording material in a thermal transfer printing system.
2. Description of the Prior Art
A thermal transfer system is known as a technique wherein a thermal energy such as from a thermal head is applied to a thermal transfer sheet having an ink layer to transfer a dye contained at least in the ink layer onto a recording sheet. Since this type of thermal transfer system is adapted for image recording, for example, according to electric signals, attempts have been made to further develop the system thereby recording color images by conversion of electric signals such as of electronic still cameras.
The recording materials which have been proposed to be used in the thermal transfer system include a sublimation-type thermal transfer sheet which has a support and an ink layer formed on the support and made of a sublimable dye and a binder resin. In this type of thermal transfer sheet, recording is effected such that the dye at portions which are heated such as by a thermal head is sublimated and transferred onto a sheet to be transferred. With the sublimation-type transfer sheet, the dye alone is transferred to the sheet to be transferred, so that it is relatively easy to make gradation recording by control of an energy inputted to a thermal head. Thus, attention has been drawn to the transfer sheet suitable for color image recording or printing.
The color image recording using the sublimation-type transfer sheet usually makes use of a thermal transfer sheet which has an ink layer containing magenta, yellow, cyan and black sublimable dyes. These sublimable dyes are thermally transferred one by one onto a material to be transferred. Accordingly, in order to attain good color image recording, the sublimation-type transfer sheet should meet requirements that the dye used has good characteristics. More particularly, the dye is readily sublimated by a thermal energy from a thermal head without involving thermal decomposition, and has a great molecule absorptivity coefficient. In addition, the dye should have a favorable hue from the standpoint of color reproduction and when printed as a cyan dye, it has preferably an absorption maximum wavelength, .lambda.max of 630 to 670 nm. Moreover, the dye should have good light fastness, dark fade resistance, migration resistance, chemical resistance and weatherability and should be easy to prepare without involving any safety and hygienic problems. Extensive developments of dyes which meet the above requirements have been proposed. For instance, indoaniline compounds which have various substituents introduced therein have been proposed as the cyan dye, for example, in Japanese Laid-open Patent Application No. 60-239289 and U.S. Pat. Nos. 4,829,047, 4,695,287 and 4,816,435.
However, these cyan dyes are not satisfactory with respect to the light fastness and the stability in a sheet to be transferred. This leads to the disadvantage that when a transfer sheet which has an ink layer containing such dyes is used to form an image, the image undergoes color change and fading, or color migration during storage.